This invention concerns a method and a device for determining the concentration of a medium other than air in an air/medium aerosol. The dielectric constant of the medium is measured using microwaves in a hollow conductor, with the aerosol passing through a neutral tube intersecting the hollow conductor. The microwave signal which propagates in the hollow conductor is subjected to attenuation and a phase shift due to the difference between the dielectric constant of the medium and that of air, the phase shift of the microwave signal being measured by taking into account the active path length (the tube in the cross section of the hollow conductor).
When determination of the H2O2 concentration is mentioned in the present patent application, it is not intended to restrict the scope in any way, but instead is given merely as an example of determining the concentration of a medium in an air/medium aerosol.
In packaging installations for aseptic packaging of foodstuffs in laminated packages, the packages are sterilized by introducing a hot air/H2O2 aerosol immediately before they are filled. This aerosol is prepared by a metering system in which the concentration of an H2O2 solution (H2O2 in water) is adjusted in a stream of air. To achieve optimal sterilization under economical conditions, it is important to measure the H2O2 concentration in the aerosol. Finally, the total quantity of H2O2 used is determined from the quantity of air used and the H2O2 concentration.
Although the quantity of air in a metering system is measured by means of suspended solid particles and then can be kept constant by adjusting the air pressure, the H2O2 concentration must be calculated. It is possible here to use measurement methods in which the measured quantity depends on the volume of H2O2, or to use measurement methods in which the measured quantity depends on the H2O2 droplet count in the aerosol.
Measurement methods based on volume have the advantage that they yield the desired measurement results directly and without conversion. In addition, a measurement method is to be provided that is capable of performing measurements by using the connecting tubes which are present anyway, without requiring direct contact with the measurement instrument and the mediumxe2x80x94for example, H2O2 which is highly corrosive. A measurement of the dielectric constant which differs from that in air is to be considered for this purpose.
It is known from W. Ho et al.: xe2x80x9cMicrowave measurements of the liquid water content of atmospheric aerosols,xe2x80x9d Journal of Applied Meteorology, vol. 13, no. 8, 1974, USA, pages 871-879, XP002058849, that the amount of liquid water in atmospheric aerosols can be measured with the help of microwave measurements. The background of these measurements is studies of the influence of the relative atmospheric humidity on the range of vision, in a fog, for example. With the known device, liquid particles are filtered by a filter out of gas passed through a pipeline and collected. This filter for liquid particles is located at the center of a cavity resonator whose resonant frequency is shifted by the presence of material having a high dielectric constant in the filter. The change in frequency in the cavity resonator is used as a measure of the concentration of the aerosol.
It is a problem with the device known from the state of the art that the concentration of the aerosol can be measured only as a semi-continuous measurement, because the filter must be dried regularly to correct for fluctuations in ambient influences. Another problem with the known device is that the only variations in concentration that can be detected with it over time are those having a time constant much greater than the time of the drying cycles of the filters. Typical intervals of approximately 20 minutes are mentioned in the state of the art. This is by far insufficient for controlling concentrations in packaging facilities, such as for aseptic packaging, for example.
German Patent No. 4,431,001 A1 discloses a method and a device for measuring the dielectric constant with a hollow conductor and a microwave transmitter and receiver. The hollow conductor is designed as a cavity resonator, having a transverse slit in its central area to arrange a test page perpendicular to the longitudinal direction of the hollow conductor. The dielectric constant of the specimen can be measured by the difference between the resonant frequencies of the cavity resonator, with and without the specimen.
The object of this invention is to design and improve upon the known device and the known method of measuring the concentration of an air/water aerosol, so they can be used with little complexity in terms of measurement technology to measure the concentration of a medium other than air in an aerosol flowing through the hollow conductor.
This objective is characterized according to this method by the fact that there is equalization to the phase shift of an aerosol with a known dielectric constant, and an average is formed from the concentration values thus determined. By reflecting the microwave signal at a short circuit in the hollow conductor, a standing wave is generated with a maximum electric field in the area of the tube, and the geometric and electric configurations are adjusted so that the phase shift is directly proportional to the concentration of the medium.
In terms of equipment, this object is achieved with a sensor and an analyzing unit. The sensor is formed by a hollow conductor, a microwave transmitter, and a microwave receiver, and has a neutral tube passing through the hollow conductor to carry the aerosol due to the fact that the microwave transmitter and receiver are arranged in the hollow conductor. One end of the hollow conductor is closed by a longitudinally displaceable, electrically conducting short circuit, and the tube is arranged in the maximum electric field of the standing wave generated in the hollow conductor. The analyzing unit has at least one preamplifier for voltage balancing and two low-pass filters for measuring the fluctuations in the concentration (Wa) of the medium and the average (Wm) concentration of the medium.
With the method according to this invention, a microwave signal propagates in the hollow conductor within which there is a tube with a flowing aerosol. The signal is scattered by the transitions from air to droplets and back, yielding attenuation in the direction of propagation of the wave. On the other hand, due to the presence of droplets having a different dielectric constant, there is a shift in transit time which can be measured from the outside as a phase shift at a constant operating frequency.
It has been found that the phase measurements and absorption measurement follow qualitatively the same pattern, so it is possible to use the measurement with the higher signal yield, the phase shift, or a combination of phase shift and absorption measurement. This leads to reduced complexity in terms of measurement technology because phase and amplitude need not be measured separately, and therefore the signal curves in the working frequency range need not be analyzed. Due to the identity of phase and amplitude curves, the relative measurement is sufficient, and there is no harm in mixing the amplitude measurement and phase measurement.
As with the state of the art previously cited, it is possible to arrange the microwave transmitter at one end of the hollow conductor and the receiver at the other end. However, in a preferred embodiment of the invention, both the microwave transmitter and the receiver are arranged at one end of the hollow conductor, and the other end of the hollow conductor is sealed by an electrically conducting short circuit. This short circuit is an electrically conducting element that fills up the entire hollow cross-section of the hollow conductor and reflects the measurement distance, so that a standing wave is formed in the hollow conductor and is altered by the aerosol. The hollow conductor preferably has a rectangular cross section, and the tube preferably runs across the longitudinal direction of the hollow conductor. In a preferred embodiment of the invention, the tube is arranged at the maximum of the electric field of the standing wave formed in the hollow conductor.
To standardize the aerosol flow conditions in the tube passing through the hollow conductor, a preferred embodiment of this invention provides for the tube to have a 180xc2x0 curvature immediately before its entrance into the hollow conductor, and for it to run preferably in a curved pipe. In this way, the sensitivity of the device over the cross-section of the tube can be standardized.
An especially simple design in terms of measurement technology is achieved according to this invention by using a conventional motion sensor as the microwave transmitter and receiver. Such motion sensors include both a microwave source and a mixer diode, whose output signals are already in the baseband. In addition, such motion sensors, which already have a flange for connecting to a hollow conductor, are currently commercially available.
An oscilloscope can be used to display the fluctuations in aerosol concentration and the average aerosol concentration, and a multimeter can be used for displaying the average alone.